1. Field of the Invention
This invention relates to a steering device for a four-wheeled vehicle, and more particularly to a four-wheel-steering device in which both the front and rear wheels are turned when the steering wheel is turned.
2. Description of the Prior Art
Conventionally, four-wheeled vehicles are steered by turning only the front wheels and the rear wheels are not positively turned though the rear wheels are caused to slightly toe in or out independently of the front wheels, according to the running condition of the vehicle.
Recently, there has been proposed a four-wheel steering device in which the rear wheels are also turned in response to operation of the steering wheel. See U S. Pat. No. 4,295,657, for example.
The four-wheel steering device greatly improves the driving stability and makes various convenient operations of the vehicle feasible. For example, when the rear wheels are turned in the direction opposite to the front wheels referred to as "reverse phase" turning) during very low speed operation of the vehicle to drive into a very limited parking space, the orientation of the vehicle body can be changed by a large amount to facilitate the parking operation. Further, the reverse phase turning of the front and rear wheels is advantageous in that the minimum turning radius of the vehicle can be minimized to facilitate U-turns, and the difference between the turning radii of the inner front wheel and the inner rear wheel can be minimized to facilitate operation of the vehicle on turning a narrow corner. Further, when the front and rear wheels are turned in the same direction (referred to as "equal phase" turning) during very low speed operation, the vehicle body can be laterally moved in translation. This is convenient for parking in a limited space.
Further when the front and rear wheels are turned in the equal phase when changing running lanes during intermediate to high speed operation of the vehicle, a lateral force acts simultaneously on the front and rear wheels to permit smooth lane-change with the yawing movement of the vehicle body being limited, whereby the operator can change running lanes safely even during high speed operation. By turning the rear wheels in the reverse phase, orientation of the vehicle body can be efficiently changed during cornering.
Further by turning the rear wheels so as to oppose them to external disturbances acting on the vehicle body such as lateral wind during straight travel of the vehicle, high running stability can be obtained during straight travel at high speed.
In the conventional steering systems in which only the front wheels are turned to steer the vehicle, the steering characteristics are adjusted to exhibit slight understeer tendency in order to ensure good stability during straight travel, and therefore, the vehicle tends to turn on a larger radius when accelerated with the turning angle of the steering wheel kept constant during cornering. In the four-wheel steering device, this tendency can be corrected by turning the rear wheels in the reverse phase. That is, in the four-wheel steering device, good cornering stability can be ensured even if the vehicle is accelerated or decelerated during cornering, by changing the turning angle of the rear wheels in response to the acceleration or the deceleration.
The four-wheel steering device is advantageous also from the roominess of the vehicle since it makes it possible to reduce the minimum turning radius of the vehicle for a given wheel base and accordingly the wheel base of the vehicle can be enlarged. Further, it provides an increased freedom in design of the vehicle body since it permits the maximum turning angle of the front wheels to be much smaller than that of the conventional two-wheel steering device.
Thus, the four-wheel steering device is advantageous from various viewpoints and is very useful.
There have been proposed various arrangements for effectively turning the rear wheels in the four-wheel steering device. For example, in the four-wheel steering disclosed in U.S. Pat. No. 4,313,514, the rear wheels are turned in the reverse phase during low speed operation of the vehicle while they are turned in the equal phase during a high speed operation. In the device disclosed in Japanese Unexamined Patent Publication No. 56(1981)-5270, the rear wheels are turned in the equal phase when the turning angle of the front wheels is small while they are turned in the reverse phase when the turning angle of the front wheels is large. Further in the device disclosed in Japanese Unexamined Patent Publication No. 56(1981)-163969, the rear wheels are turned in proportion to the turning angle of the front wheels when the turning angle of the front wheels is smaller than a predetermined value, while the rear wheels are kept at a predetermined turning angle independent of the turning angle of the front wheels when the turning angle of the front wheels is not smaller than the predetermined value.
These four-wheel steering devices are directed to turning the rear wheels in the desirable direction based on the empirical rule that in most cases where orientation of the vehicle body is to be changed by a large amount, the vehicle speed is low or the turning angle of the front wheels is large, and in most cases where a slight lateral movement of the vehicle body is desired, the vehicle speed is high or the turning angle of the front wheels is small.
However, as a matter of fact, completely satisfactory driving performance and running stability cannot be obtained when the vehicle speed and the turning angle of the front wheels are only taken into account independently of each other. For example, even if the turning angle of the front wheels is fixed at a given angle and the rear wheels are turned by a desirable angle corresponding to the given turning angle of the front wheels during cornering, the vehicle tends to go out of the originally intended course outwardly or inwardly when the vehicle is accelerated or decelerated during the cornering. This is because the centrifugal force (lateral force) acting on the vehicle body changes with the vehicle speed. In order to prevent the vehicle from going out of the originally intended course, the ratio of the turning angle of the rear wheels to that of the front wheels (referred to as the "turning angle ratio" hereinbelow) must be changed with change in the vehicle speed. Accordingly the turning angle ratio is preferred to be changed according to the vehicle speed. Further, during intermediate to high speed operation of the vehicle, it is preferred that the turning angle ratio be increased in the equal phase as the vehicle speed is increased so that the lateral acceleration G is increased to permit smooth lane-changing, thereby improving the driving performance. On the other hand, it is preferred that the turning angle ratio be reduced as the vehicle speed is lowered from a high speed to an intermediate speed, to reduce the lateral acceleration G.
In the case of the four-wheel steering device disclosed in U.S. Pat. No. 4,313,514 in which the rear wheels are turned in the reverse phase during low speed operation of the vehicle, there is a danger that when a vehicle which has been parked near a wall, for example, is started with the steering wheel turned sharply, the rear part of the vehicle could collide against the wall since the rear wheels are turned in the reverse phase at a low speed. Further when the vehicle speed is abruptly reduced from a speed in the intermediate-to-high region by abrupt braking, the rear wheels could be turned in the reverse phase to cause the vehicle to make a dangerous sharp turn. In order to overcome these problems, there has been proposed a four-wheel steering device in which the turning angle ratio is made zero, i.e., the rear wheels are not turned, during low speed operation of the vehicle. See Japanese Unexamined Patent Publication No. 56(1981)167562, for example. However, this approach is disadvantageous in that the four-wheel steering device behaves in the same manner as the ordinary steering device in the low speed region and accordingly the merit of the four-wheel steering device in making U-turns at a low speed or changing the orientation of the vehicle by a large amount at a low speed cannot be obtained.